1. Field of the Invention
The present invention is directed toward a cementing tool, a casing string equipped with a cementing tool, and methods of cementing such a casing string. Particularly, the cementing tool is provided with a rupture disc assembly that upon rupture permits cement to flow from the interior of the casing string through the tool sidewall and into the annulus defined by the casing string and downhole formation into which the casing string is run. The cementing tool permits obstructions or voids within the annulus to be bypassed during cementing operations, and allows for multiple-stage cementing operations to be conducted. Further, the cementing tool, if activated during cementing operations, restores the structural integrity of the casing string that might otherwise be lost through the use of other tools or processes.
2. Description of the Prior Art
Surface casing is typically the first casing string run and fully cemented in a well. Surface casing protects fresh water-bearing sands or formations from vertical migration of well fluids that might otherwise contaminate the fresh water carried by these formations. Often too, the well blow out preventer, which is the last line of defense against an uncontrolled well, is secured to the surface casing. Further, surface casing is used to hang off the next string of casing that is run into the well. Given the many functions of surface casing, it is important for the surface casing to be well supported in order to prevent buckling and damage when loaded in this manner.
The purpose of cementing the surface casing is to have a competent sheath of cement to both support and seal around the casing. During cementing operations, cement is introduced into the annulus created between the casing and the formation through which the casing is run. Cement can be introduced into the annulus in a number of ways. One method is “top job” approach wherein cement is directly injected into the annulus from the surface using one or more small diameter pipes pushed down into the annulus. This method may be useful in cementing shallow casing strings, but is not always reliable in that un-cemented pockets can be left in the annulus. Another method involves the circulation of cement down through the center of the casing string and back toward the surface through the annulus. When successfully completed, this method provides a higher degree of confidence that un-cemented pockets have been avoided or minimized. However, the annulus can become obstructed, such as with a collapsed portion of a loose formation which blocks the flow of cement through the annulus. In other instances, cement may be lost from the annulus into the well formation due to the high porosity of the rock or sand that the well bore is drilled through. This loss prevents the cement from reaching the surface and is known as lost circulation or lost returns. In these instances, the casing would need to be perforated above the obstruction or region of lost circulation so that a new flow path for cement into the annulus can be established. This is undesirable as it requires compromising the casing integrity.
Another solution has been proposed involving the use of differential valve (DV) tools. These tools have largely been used as a part of a multistage cementing operation. These tools are typically run where the cementing is planned to be placed in multiple lifts in a single string of pipe. The bottom section of casing is cemented normally. Then the tool is opened and drilling mud is circulated. After the bottom stage of cement has been set sufficiently, the top stage is cemented through the DV tool. These tools are disadvantageous in that the cementing must be performed in stages, rather than in a single pour, thus adding additional operating time to the cementing process. Further, these tools tend to be expensive and most require some kind of actuation operation, and then be drilled out once the cementing stage is completed.